The invention has been developed in particular in view of the application on agricultural tractors of the type comprising:
a pair of rear wheels rigidly connected to a main frame, and a pair of front wheels carried by a front axle mounted for oscillatory movement about a longitudinal axis of the tractor, PA1 a control cab mounted on the main frame, and PA1 suspension means interposed between the cab and the main frame.
Agricultural machines often operate in non-levelled working conditions, due to either ground configuration (e.g. hilly ground) or to specific operations such as furrow plowing in which even on level ground the tractor is transversally tilted.
In all these conditions, the operator loses a major part of the comfort which the cab can provide on horizontal ground. In fact, longitudinal inclinations of the tractor can be compensated only by forces applied with the legs on the floor and with the arms on the driving wheel, whereas transversal inclinations are compensated because the operator leans on the seat armrests. These positions can be very troublesome for the operator after a few working hours both for lower and upper limbs fatigue and for stiffening of the hips which continuously rest on the lateral armrests. This produces, in addition to a concentration drop, also a reduced productivity of the tractor.
U.S. Pat. No. 4,018,296 shows a self-levelling tractor which automatically levels the main frame of the tractor with respect to the front and rear axle and to the tractor wheels. The tractor axles are mounted for oscillatory movement about a common axis which extends in a longitudinal direction below the main frame and a pair of hydraulic cylinders are arranged between the main frame and the rear axle. A mercury levelling unit is mounted to the main frame and actuates the hydraulic cylinders to pivot the main frame to horizontal when the rear axis and its wheels are tilted because of the terrain angle.
The structure disclosed in U.S. Pat. No. 4,018,296 requires heavy modifications of the tractor structure for rendering the rear axle pivotable with respect to the main frame.